Semiconductor superlattice devices are well known in the art and comprise portions of light emitting devices, light modulators etc. A compositional superlattice is a periodic array of ultra- thin layers of two different semiconductors in alternation. Each layer is about a hundred Angstroms thick to enable quantum effects to govern their electronic properties. The periodic alternation of layers gives rise to a periodic variation of electric potential, and each layer with the smaller band gap produces what is called a quantum or potential well. The most widely used superlattice devices employ thin layers of GaAs interspersed between layers of AlGaAs. Typically, the thickness of each GaAs layer is approximately 50-100 Angstroms with the AlGaAs layer thickness being in the same range.
Recently, others have found that the excitonic optical absorption and refractive index characteristics of a superlattice multiple quantum-well (MQW) device can be altered by the application of electric fields, either perpendicular to the superlattice planes or parallel thereto (referred to as the Stark effect). A comprehensive study of these phenomena can be found in "Electric Field Dependence of Optical Absorption Near the Band Gap of Quantum-Well Structures", Miller et al. Physical Review B, Vol. 32 No. 2, pp. 1043-1060, 15 July, 1985. FIG. 1 herein illustrates a device structure used by Miller et al. to investigate the effects of a perpendicular electric field on the optical properties of an MQW structure.
In FIG. 1, an MQW active region 10 comprises a plurality of GaAs/AlGaAs layers with each GaAs layer being 29 Angstroms in thickness and each AlGaAs layer being 69 Angstroms. On either side of MQW region 10 is a pair of superlattice buffer regions 12 which are, in turn, sandwiched between a pair of p and n-type contact layers 14. A further AlGaAs p+ contact region 16 forms a support for contact metallization 18. An optical beam 20 enters the semiconductor structure via opening 22 in metallization layer 18. On the opposite side of the structure, an AlGaAs n+ region is employed as an etch stop. A further GaAs n+ buffer region is positioned adjacent a GaAs substrate n+ region which, in turn, provides the support for contact metallization 30. The optical beam exits from region 24, as indicated by arrow 32.
While the Miller et al. paper describes the effects of applied, perpendicular and parallel electric fields on the optical properties of excitons confined in the MQW region 10, it is the effect of the perpendicularly applied electric field which is most interesting. Referring to FIG. 2, there is reproduced from Miller et al., a plot of absorption spectra at various electric field values for a perpendicularly applied field to the structure shown in FIG. 1. The perpendicular electric field is created by applying an appropriate DC bias between contacts 18 and 30. Curve 40 indicates the absorption coefficient as function of the incident photon energy for the structure of FIG. 1 with an applied electric field of approximately 1 .times.10.sup.4 V/cm. Curves 42 and 44 show changes in the absorption coefficient for MQW layer 10, as the applied field is increased to 4.7 .times.10.sup.4 V/cm and 7.3 .times.10.sup.4 V/cm, respectively. The zero lines 46 and 44 for curves 42 and 44 are displaced from zero line 48 for clarity's sake.
As the applied electric field is increased, there is a substantial decrease in photon energy of the peak absorption coefficient for MQW layer 10. Thus, if an optical beam exhibiting a photon energy of approximately 1.45 eV is applied to the device of FIG. 1, and the electric field is varied between the values for curves 40 and 42, the absorption of that beam will vary significantly for the two different field values. High field values are required to achieve the significant changes in absorption coefficient due to the fact that the optical beam interacts with excitons in a very narrow thickness of MQW region.
Further investigation of a structure similar to that of Miller et al. is described in "Field Effects o the Refractive Index and Absorption Coefficient in AlGaAs Quantum Well Structures and Their Feasibility for Electrooptic Device Applications", Kan et al., IEEE Journal of Quantum Electronics, Vol. QE-23, No. 12, Dec. 1987, pp. 2167-2179 Kan et al. investigated the changes of both absorption coefficient .alpha. and refractive index n for a superlattice device similar to that shown in FIG. 1. Those results are reproduced in FIGS. 3 and 4.
In FIG. 3, changes in both refractive index and absorption coefficient are plotted against wavelength. Curve 50 shows the variation of refractive index n over various wavelengths when no electric field is applied Dotted curve 52 shows changes in refractive index n when the applied field is increased to 6 .times.10.sup.4 V/cm. Curves 54 and 56 in FIG. 3 show the corresponding changes in absorption coefficient .alpha. for identical changes in the electric field value, as plotted against wavelength of the applied light.
In FIG. 4, the refractive index variation .DELTA. n and absorption variation .DELTA..alpha. is plotted against wavelength. It can be seen that the change varies in .DELTA. n varies significantly between approximately 843 nm to 850 nm and the absorption variation .DELTA..alpha. varies significantly between approximately 847 nm to 852 nm.
The results of Kan et al. show, for selected wavelengths, that both the absorption coefficient and refractive index of a superlattice MQW structure can be significantly altered by the application of a perpendicular electric field For instance, as shown by curves 50 and 52, at 850 nm the refractive index varies significantly when an applied perpendicular electric field is varied. Kan et al. suggest the use, as a modulator, of a structure similar to that in FIG. 1. As with Miller et al. a high value perpendicular electric field is required to accomplish the desired modulation due to the short interaction length between MQW region and the optical beam.
A further application of this type of structure is considered in "Quantum-Well Charge Coupled Devices for Charge-Coupled Device Addressed Multiple-Quantum-Well Spatial Light Modulators", Goodhue et al., Journal of Vacuum Science Technology, B4(3), May/June 1986, pp. 769-772. Goodhue et al. describe a spatial light modulator which employs an MQW region, on which a charge-coupled-device (CCD) shift register has been constructed. The CCD shift register has interspersed opaque and transparent electrical contacts. Thus, when a signal charge packet in the channel is transported under the transparent electrode, it modifies the magnitude of the perpendicular electric field into the MQW layers which, in turn, alters its absorption characteristics. Light passing through the transparent CCD electrode and into the MQW is thus selectively, locally absorbed, depending upon the voltage present on the transparent electrode By necessity, the CCD modulation structure covers a significant portion of the face of the device and restricts the amount of surface area available for light modulation In addition, it requires 3-phase clock circuitry and associated lithography to transport charge packets.
Another type of prior art optical modulator is the acousto-optic modulator that employs a surface acoustic wave (SAW) to modulate an optical signal. Such a structure is described in "Correlator Based on an Integrated Optical Spatial Light Modulator", Verber et al., Applied Optics, Vol. 20, No. 9, 1 May, 1981, pp. 1626-1629. Verber et al. employ a SAW structure on lithium niobate to induce electric fields therein, which fields diffract an optical beam. The modulator divides a single broad beam, incident at the Bragg angle, into two angularly separated beams. The SAW is modulated with a binary data pattern, so that optical beam segments which encounter either two gratings or no gratings, exit at the same angle they entered, while beams which encounter only one grating are deflected by twice the Bragg angle. The effect used in Verber's device is not based on confined excitonic phenomena taking place only in MQW structures.
A further development in SAW devices is disclosed in U.S. Pat. No. 4,633,285 to Hunsinger et al. and in "Heterojunction Acoustic Charge Transport Device Technology", Cullen et al., 1988 Ultrasonics Symposium, pp. 135-143, 1988, IEEE. Both Hunsinger et al. and Cullen et al. employ the piezoelectric properties of a GaAs/AlGaAs semiconductor structure to configure a SAW device. Both show the use of a buried channel of GaAs disposed between confining AlGaAs layers. A surface acoustic wave is induced in the buried channel and electric charges are injected therein via a Schottky barrier contact. The surface acoustic wave carries the injected charges along the channel to a sensing electrode. In addition to being employed as a delay line, Hunsinger et al. also disclose the use of the SAW structure as an image detector, such that light falling upon the structure causes charges to be injected into the buried channel and carried along by the surface acoustic wave.
To the inventors' knowledge, no prior art has combined the benefits accruing from the use of a surface acoustic wave device and an MQW structure hosting confined excitons to perform optical modulation.
Accordingly, it is an object of this invention to provide an improved SAW-based, acousto-optic light modulator.
It is a further object of this invention to provide an improved acousto-optic light modulator which makes use of electric-field induced variations in the optical properties of an MQW structure.
It is still another object of this invention to provide an acousto-optic modulator particularly adapted for integration into monolithic semiconductor structures.